Touch sensitive display device

ABSTRACT

In a touch sensor the change in electrical characteristics (impedance, piezo-voltage) of spacing elements ( 13,14,15,25 ), provided with an conducting, resistive or piezo-electric layer ( 15, 25 ), are measured to determine the sensing area.

The invention relates to a touch sensitive display device comprising amultiple of picture elements between two substrates, having spacingmeans between the substrates and means for applying driving voltages toat least one of said picture elements together with means for monitoringthe electrical characteristics of at least one of said picture elementsand sensing a change in said electrical characteristics.

The display device is for instance a liquid crystal display device.Liquid crystal display devices have found widespread use in the computerindustry and in handheld devices ranging from mobile telephones andprice tags to palm top computers and organizers. Also the combinationwith a touching device such as a stylus has found widespreadapplications, while also a need for ways of providing input via thedisplay screen is felt.

U.S. Pat. No. 5,777,596 describes a touch sensitive liquid crystaldisplay device that allows putting input into the associated device(e.g. a computer) by simply touching the display screen with a finger, astylus or a pen. The device continuously compares the charge time of theliquid crystal display elements (picture elements) to a reference valueand uses the result of the comparison to determine which elements arebeing touched.

One of the problems in said touch sensitive liquid crystal displaydevice resides in restoring the right image after sensing. This is dueto the fact that a blinking line is used which represents the switchingof all picture elements in a row between two extreme states. When theblinking line reaches a certain row touching is detected by measuringthe charging time of the picture elements. After measuring the pictureelements are provided with adequate voltages to display the right image.In a similar way sensing by means of a blinking spot is disclosed inU.S. Pat. No. 5,777,596.

Such blinking however is visible on the display (artifacts)

Moreover, if a reflective display device is used, internal DC biasvoltages may be present whereby charging differs for writing odd or evenframes. in DC driving methods (low power liquid crystal displays,electrophoresis) no inversion occurs so the method cannot be used at allthere.

The invention has among others as its goal to overcome these objections.

It has as a further goal to introduce more functionality into the touchsensitive liquid crystal display device.

To this end in a touch sensitive display device according to theinvention the spacing means are part of said means for monitoring theelectrical characteristics. Said electrical characteristics may becapacitive, (non-linear) resistive or piezo-electric characteristics.

In fact the invention provides a method of non-interactive measuring;the method of measuring does not interfere with the providing of drivingvoltages to the picture elements.

This does not only overcome the problem of providing blinking signalsbut also, in certain embodiments offers new possibilities of touchsensing such as

-   i) sensing touch inputs at different places on the display screen-   ii) disabling part of the display screen for touch sensing.

Both possibilities offer substantial advantages both in computer andtelecommunication applications.

Sensing touch inputs at different places on the display screen offerpossibilities such as detecting the impact of fingers or pencils ondifferent places of the display screen. This is a useful item in e.g.flat screen (computer) devices in which the keyboard functions have beenrealized as touch functions on the screen. It is for example possible todetect simultaneous touching of CRTL, ALT and DEL pressing; in e.g.drawing programs the simultaneous touching of two points with a pen mayimmediately display a straight line, while at the same time via a thirdtouching (area) this line may receive a certain curvature or hatching orfor implementing gaming applications etc.

Disabling part of the display screen for touch sensing may be used in acellular phone preventing the read out from being disturbed. On theother hand data input, e.g. obtained via the Internet may preventcertain parts (displaying logos) to be disturbed or disable certain menubars for unauthorized users.

Dependent on the application sensing itself may be performed indifferent ways, varying from a simple four-point measurement tomeasuring a current, a change in voltage or a change in frequency.

In one embodiment the spacing means at least have a conducting part. Forsome methods of sensing it is advantageous if the conducting part of thespacing means forms a grid.

In other embodiments the spacing means comprise a (non-linear) resistiveor a piezoelectric part. Also in this case it may be advantageous if theresistive or a piezoelectric part of the spacing means forms a grid.

One of the solutions according to the invention is to ensure that manypixels along the column (or row) are sensed at the same moment. In thiscase, the touch signal will increase with the number of pixels beingsensed, whilst the background impedance will remain constant. In thisway the signal to noise ratio will increase.

To this end in a first embodiment of a touch sensitive display devicethe means for monitoring impedance monitor at least one row of pictureelements, while in a second embodiment the means for monitoringimpedance monitor at least one column of picture elements. Alsomonitoring of the impedance of a block of picture elements is possible.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 schematically shows a touch sensitive (liquid crystal) displaydevice,

FIG. 2 shows plan views of a part of a touch sensitive (liquid crystal)display device according to the invention at a bottom plate and at a topplate,

FIG. 4 shows cross-sections along lines III^(a)-III^(a) andIII^(b)-III^(b) in FIG. 2, while

FIG. 4 shows a conductive grid for use in the embodiment of FIGS. 2,3and

FIG. 5 shows plan views of a part of a further touch sensitive (liquidcrystal) display device according to the invention at a bottom plate andat a top plate,

while FIG. 6 shows cross-sections along lines VI^(a)-VI^(a) andVI^(b)-VI^(b) in FIG. 5 and

FIGS. 7-9 show further embodiments of a part of a touch sensitive(liquid crystal) display device according to the invention.

The Figures are diagrammatic and not drawn to scale. Correspondingelements are generally denoted by the same reference numerals.

FIG. 1 is an electric equivalent circuit diagram of a part of a touchsensitive display device 1 to which the invention is applicable. Itcomprises in one possible embodiment (one mode of driving, called the“passive mode”) a matrix of pixels 8 defined by the areas of crossingsof row or selection electrodes 7 and column or data electrodes 6. Therow electrodes are consecutively selected by means of a row driver 4,while the column electrodes are provided with data via a data register5. To this end, incoming data 2 are first processed, if necessary, in aprocessor 3. Mutual synchronization between the row driver 4 and thedata register 5 takes place via drive lines 9.

In another possible embodiment (another mode of driving, called the“active mode”) signals from the row driver 4 select the pictureelectrodes via thin-film transistors (TFTs) 10 whose gate electrodes areelectrically connected to the row electrodes 7 and the source electrodesare electrically connected to the column electrodes. The signal which ispresent at the column electrode 6 is transferred via the TFT to apicture electrode of a pixel 8 coupled to the drain electrode. The otherpicture electrodes are connected to, for example, one (or more) commoncounter electrode(s). In FIG. 1 only one thin-film transistor (TFT) 10has been drawn, simply as an example.

FIG. 2 shows plan views (FIGS. 2 a, 2 b) and FIG. 3 shows cross-sectionsalong lines III^(a)-III^(a) and III^(b)-III^(b) in FIG. 2 a of a part ofa touch sensitive liquid crystal device having a bottom substrate 11 andan upper substrate 12. The touch sensitive liquid crystal device haspicture electrodes 8 on the bottom substrate 11. The picture electrodesare surrounded by, in this case rectangular, spacer parts 14 for exampledeposited (by means of e.g. photolithographical techniques) on saidbottom substrate 11. On the other substrate 12, bearing an electrode 20,distributed spacer parts 15 are deposited (prepared for example by meansof e.g. photolithographical techniques) in such a way that afterbringing the substrates together, to obtain a defined cell gap of theliquid crystal device, filling openings 21 remain.

According to the invention a conducting spacer part 13 is introducedbetween the spacer parts 14 and the distributed spacer parts 15, in thisembodiment having substantially the same layout as the rectangularspacer parts 14. A good material for the conducting spacer parts 13 isfor example one of the metals aluminum or silver.

Capacitive touch sensing by touching the change of capacitance betweene.g. electrode 20 and the conducting spacer parts 13 is realised bydetermining the AC impedance of the grid of conducting spacer parts 13at a certain number of points, for example at the four corners (FIG. 4)by means of voltage or current sensors 22. By touching the screen, thecapacitance to the grid locally increases. This will generate adifferent signal at the 4 corners, depending upon the distance of thetouch position to the sensors. In this way the position co-ordinates aredetected.

If only a limited touch sensing function in one direction is required(for example in combination with a scrolling menu feature) capacitivetouch sensing with a conducting spacer part is straightforwardlyimplemented. In such an embodiment the spacers are structured in theform of strips, running across the entire display. Filling of thedisplay (for example with liquid crystal material) is readily achieved,as open channels are automatically available between the structuredspacers.

Integral capacitive touch sensing is realized again in any of the knownmethods and the position co-ordinate identified by detecting which ofthe spacer lines registers the largest signal.

In general the pixel capacitance of one pixel is overshadowed by thecapacitance of other pixels (in passive matrix), cross overs and straycapacitances (active matrix) in the columns and rows. This reduces thesensitivity.

One solution to this is to ensure that many pixels along the column 6(or row 7) are sensed at the same moment. In this case, the touch signalwill increase with the number of pixels being sensed, whilst thebackground capacitance will remain constant. In this way the signal tonoise ratio will increase. In a preferred embodiment, the touch sensingprocedure will involve many rows 7 being addressed at the same time(active matrix) or many columns 8 being connected to increase the touchsignal.

FIGS. 5 and 6 show a further embodiment of a touch sensitive displaydevice according to the invention based on capacitive detection methods.The spacing elements 14, 15 are structured in the form of strips, whichare located along the entire length and width of the display. Bothsubstrates are provided with these spacing elements, but theirorientation is mutually perpendicular. On one substrate e.g. the uppersubstrate 12, the spacing elements contain an insulating spacer part 15and conducting spacer parts 23 like metal strips. On the other, bottomsubstrate 11 the spacing elements contain conducting spacer parts 13like metal strips with insulating spacer parts 14, 14′ on both sides.

The display device is finalized, in a method known in the art, byaligning and contacting the two substrates. In this way open channelsare realized for filling of the cells with liquid crystal material. Inthis way, four electrically conducting electrodes are realized—pictureelectrode 20 (e.g. a part of a row)—conducting spacer part 23 e.g. a setof strips—conducting spacer parts 13 e.g. a set of strips—pictureelectrode 8 (e.g. a part of a column).

Capacitive touch sensing is performed by one of the methods known in theart. The position co-ordinates are identified by detecting capacitancechanges between spacer grid 13 and the column (picture) electrode 8 (C1,x-direction) and spacer grid 23 and the row (picture) electrode 20 (C2,y-direction). In this way the touch position is determined without anyinterference with the working of the display device itself (i.e. it isno longer necessary to drive display pixels for displaying images andfor detecting touch information separately).

In a special embodiment the insulating spacer parts 14 between theconducting spacer parts 13, 23 comprise deformable or compliantinsulating material, leading to a capacitance C3, which now varies withtouching. it is now also possible to determine the touch position bymeasuring the change in capacitance C3. This has advantage that themeasurement will now be much less dependent upon the changes ofdielectric constants of liquid crystal material due to its switchingbehavior by measuring the change in capacitance C3 the position ofsensing is determined by a change

Preferably the capacitances C1, C2 and C3 are measured separately. Thetouch position can be more accurately determined in this case and falsetouch readings be excluded more easily (C1, C2 and C3 all need torespond to register a touch event).

In the embodiment of FIG. 7 the conducting spacer grid 13 is situated ona thicker continuous structured spacer part 14 on substrate 11, whilethe second substrate 12 is provided with thinner structured spacer parts15. By creating such a small distance between electrode(s) 20 and thestructured spacer part 14, touch sensing can be carried out by causing alocal short circuit between the (exposed portion of the) conducting grid13 and the electrode 20. Detection can be carried out by means ofresistive touch sensing methods known per se e.g. by sequentiallyapplying voltages in two directions and measuring the voltage detectedat the touch position and determining the position by resistivedivision. This is applicable both to active matrix displays (as theyhave a continuous counter electrode) and to passive matrix displays e.g.by shorting the electrodes on (top) substrate 12 and detecting signalson the conducting grid 13 by means of said resistive touch sensingmethods.

Preferably, for displays with structured electrodes 20 on the uppersubstrate 12, these electrodes 20 are used to determine the touchposition (the co-ordinate) in one direction (the x-direction) bydetermining which electrode was shorted and a similar resistive divisionapproach is used to determine the touch position (the co-ordinate) inthe other direction (the y-direction). This has the advantage of highaccuracy without further requirements for temperature compensation ofthe sensor (as known per se for prior art resistive touch sensors).

FIG. 8(a) together with its electrical equivalent in FIG. 8(b) (in thiscase of single liquid crystal picture element) show a further embodimentin which the spacing element comprises a non-linear resistive element 25and an insulating part 13 between electrodes 8, 20 which are in thisexample column and row electrodes of a (passive) matrix. Non-linearpressure sensitive resistance material (which drastically reduces itsresistance when pressure is applied) is known from e.g. WO 99/38173.

If no pressure is applied the capacitance C_(d) is determined byinsulating layers 13, 25 and will have a low value. When pressure isapplied C_(d) will rise, since it is only determined by insulating layer13.

The touch position will be directly measured in x and y directions (ascolumn parts 20 and row parts 8 of the spacers will be contacted atthese positions) by a (schematically shown) measuring device 22 (currentor voltage detection circuit).

In the embodiment of FIG. 8 the insulating part 13 between electrodes 8,20 may even be deleted as shown in FIGS. 9(a) and 9(b). FIG. 9(c)finally shows how a piezoelectric spacing part is used, which behaves asa variable voltage source 25′. Pressure on the piezoelectric spacerswill directly lead to an output voltage signal, which is used todetermine the touch position. For x,y touch sensing, this is easilyaccomplished by using a continuous mesh of the piezoelectric element andpositioning four sensors 22 at the corners of the mesh (similar to thescheme of FIG. 4). Alternatively more sensors can be employed to improvethe accuracy of sensing. In stead of a grid sets of strips can be used,similar to the embodiment of FIGS. 5, 6.

The protective scope of the invention is not limited to the embodimentsdescribed, while the invention is also applicable to other displaydevices, for example, plasma displays, and other display devices usingspacing devices (display devices on electrophoretic effect,electrowetting, electotochrome effects or foil displays).

Alternatively, flexible substrates (synthetic material) may be used(wearable displays, wearable electronics).

The invention resides in each and every novel characteristic feature andeach and every combination of characteristic features. Referencenumerals in the claims do not limit their protective scope. Use of theverb “to comprise” and its conjugations does not exclude the presence ofelements other than those stated in the claims. Use of the article “a”or “an” preceding an element does not exclude the presence of aplurality of such elements.

1. A touch sensitive display device comprising a multiple of pictureelements between two substrates, having spacing means between thesubstrates and means for applying driving voltages to at least one ofsaid picture elements together with means for monitoring the electricalcharacteristics of at least one of said picture elements or theelectrical characteristics of the spacing means and sensing a change insaid electrical characteristics the spacing means being part of saidmeans for monitoring the electrical characteristics.
 2. A touchsensitive display device as claimed in claim 1 with means for monitoringthe impedance of at least one of said picture elements and sensing achange in said impedance the spacing means being part of said means formonitoring the impedance.
 3. A touch sensitive display device as claimedin claim 1 in which the means for sensing the change in said impedancemeasure impedances of different groups of picture elements substantiallysimultaneously.
 4. A touch sensitive display device as claimed in claim1 in which the spacing means at least have a conducting part.
 5. A touchsensitive display device as claimed in claim 4 in which the conductingpart of the spacing means is in the form of a grid or in the form ofsets of strips.
 6. A touch sensitive display device as claimed in claim1 in which the spacing means at least have a nonlinear resistive part.7. A touch sensitive display device as claimed in claim 6 in which thenonlinear resistive part of the spacing means is in the form of a gridor in the form of sets of strips.
 8. A touch sensitive display device asclaimed in claim 1 in which the spacing means comprise a piezoelectricpart.
 9. A touch sensitive display device as claimed in claim 8 in whichthe nonlinear resistive part of the spacing means is in the form of agrid or in the form of sets of strips.
 10. A touch sensitive displaydevice as claimed in claim 1 in which the means for monitoring theelectrical characteristics monitor at least one row of picture elements.11. A touch sensitive display device as claimed in claim 1 in which themeans for monitoring the electrical characteristics monitor at least onecolumn of picture elements.
 12. A touch sensitive display device asclaimed in claim 1 in which the means for monitoring the electricalcharacteristics monitor a block of picture elements.